1. Field of the Invention
The present invention is directed to correcting beam hardening in an X-ray computed tomography apparatus.
2. Description of the Prior Art
As a consequence of the spectral dependency of the beam attenuation behavior in an irradiated object, a shift of the average or mean energy of the X-radiation emerging from a transirradiated body toward higher energy values occurs given polychromatic X-radiation. This effect is referred to as beam hardening. In computed tomography, beam hardening causes gray scale deviations in the reconstructed image of the body compared to the theoretical case of linear, spectrally independent beam attenuation. These gray scale deviationsxe2x80x94or beam hardening artifactsxe2x80x94in the reconstructed image interfere with the diagnostic content of the image and can lead to misinterpretations in the worst case.
Numerous approaches are proposed in the literature for correcting image artifacts caused by beam hardening. For example, one approach disclosed in U.S. Pat. No. 4,709,333 is known as polynomial correction. Using an attenuation value obtained by measurement that indicates the actual beam attenuation of a body affected by beam hardening, a corrected attenuation value that is employed as the basis of the image reconstruction is calculated by inserting this attenuation value into a suitable polynomial identified in a calibration phase.
An object of the present invention is to provide a computed tomography apparatus which is operable to effectively correct for beam hardening.
This object is inventively achieved in an X-ray computed tomography apparatus having an X-ray radiator, a radiation filter arrangement arranged in the beam path of the X-rays emitted by the X-ray radiator, a detector arrangement that detects the X-rays that pass through a patient under examination and provides a set of measured intensity values for each slice projection, each of these measured intensity values being representative of the intensity of the detected X-rays in a respective projection sub-region of the slice projection, and has an electronic evaluation unit that determines an overall attenuation value for each of the measured intensity values that is representative of the actual overall attenuation of the X-rays in the respective projection sub-region effected by the radiation filter arrangement and the patient, and that determines a patient attenuation value corrected for beam hardening for each of the overall attenuation values that is representative of the theoretical, linear attenuation of the X-rays by the patient in the respective projection sub-region.